1. Field of the Invention
The present invention relates to a polypropylene-based resin composition, a process for producing the same and an injection molded article made of the same. More particularly, the present invention relates to a polypropylene-based resin composition which, when made into a molded article, has a low generation of flow marks, namely, a high die swell, and excellent appearance such as little generation of granular structures (fish eyes), and is excellent in balance of rigidity and toughness; a process for producing the same and an injection molded article made of the same.
2. Description of Related Arts
Polypropylene-based resin compositions are materials excellent in rigidity, impact resistance and the like, and are utilized in applications as molded articles such as automobile interiors and exterior materials, electric appliance housings and the like. It is conventionally well known that, among these polypropylene-based resin compositions, polypropylene-based resin compositions containing a propylene-ethylene block copolymer, for example, polypropylene-based resin compositions composed of a propylene-ethylene block copolymer and a propylene homopolymer, or of different two or more propylene-ethylene block copolymers, are excellent in rigidity, impact resistance and the like.
For example, JP07-157626A discloses a thermoplastic resin composition containing a propylene-ethylene block copolymer composed of a propylene homopolymer phase and a propylene-ethylene copolymer phase obtained by multi-stage polymerization, and polyolefin rubber. The propylene-ethylene block copolymer is composed of a propylene-ethylene random copolymer phase (i) having an ethylene content of from 5 to 50% by weight and an intrinsic viscosity of from 4.0 to 8.0 dl/g, and a propylene-ethylene block copolymer phase (ii) having an ethylene content of over 50% by weight and 98% by weight or less and an intrinsic viscosity of 2.0 dl/g or more and less than 4.0 dl/g, and it is disclosed that a thermoplastic resin composition having an extremely high ductility is obtained.
JP07-157627A discloses a thermoplastic resin composition containing a propylene-ethylene block copolymer obtained by multi-stage polymerization and polyolefin-based rubber. The propylene-ethylene block copolymer is composed of a propylene-ethylene block copolymer in which the propylene-ethylene random copolymer phase (iii) has an intrinsic viscosity of from 4.0 to 8.0 dl/g, and a propylene-ethylene block copolymer in which the propylene-ethylene block copolymer phase (iv) has an intrinsic viscosity of 2.0 dl/g or more and less than 4.0 dl/g, (excluding a propylene-ethylene block copolymer having a propylene-ethylene random copolymer phase having an intrinsic viscosity of from 4.0 to 8.0 dl/g and an ethylene content of from 5 to 50% by weight and a propylene-ethylene block copolymer having a propylene-ethylene random copolymer phase having an intrinsic viscosity of 2.0 dl/g or more and less than 4.0 dl/g and an ethylene content of over 50% by weight and 98% by weight or less), and it is disclosed that the thermoplastic resin composition having extremely high ductility is obtained.
JP07-233305A discloses a polypropylene resin composition having an improved melt index, composed of a polypropylene, a inorganic filler and a metal soap of a zinc salt of a fatty acid having 15 to 20 carbon atoms. As the polypropylene, block copolymers containing a propylene-ethylene copolymer part having a [xcex7] of 2 to 6 dl/g and a propylene-ethylene copolymer part having a [xcex7] of 7 to 15 dl/g are described, and as a preferable embodiment, those composed of at least two kinds of block copolymers are described. Further, it is described that a polypropylene resin composition manifesting no problematical lowering in impact strength, rigidity, heat resistance or the like, having improved flowability and suitable for producing a molded article thinned in thickness, and showing little discoloration to yellow or pink caused by pelletizing and little bleeding of low molecular weight compounds on the surface of a mold and the surface of an injection molded article, is obtained.
In addition, a propylene-ethylene block copolymer used due to excellent rigidity, impact resistance and the like is recently produced by a continuous gas phase polymerization method because of a simple production process and production at low cost. However, the propylene-ethylene block copolymer produced by the gas phase method generally has problems of poor appearance as a molded article due to generation of granular structures when the intrinsic viscosity of a propylene-ethylene random copolymer portion is set at a high level.
For solving such problems on appearance, for example, JP07-286075A discloses a propylene polymer composition composed of a propylene polymer produced by a continuous polymerization method in which the 23xc2x0 C. n-decane-soluble component content is 0% by weight or more and less than 15% by weight and the 23xc2x0 C. n-decane-soluble component contains a structural unit derived from ethylene in an amount of 30 to 60 mol % and has an intrinsic viscosity [xcex7] of 3 to 7 dl/g, and a propylene block copolymer produced by a batch-wise solvent polymerization method or continuous solvent polymerization method in which the 23xc2x0 C. n-decane-soluble component content is 15 to 40% by weight and this 23xc2x0 C. n-decane-soluble component contains a structural unit derived from ethylene in an amount of 30 to 60 mol % and has an intrinsic viscosity [xcex7] of 5 to 12 dl/g, and it is described that a propylene polymer composition having excellent rigidity and impact resistance and capable of forming a molded article without generating granular structures on appearance is obtained, however, there is desired a further improvement regarding generation of granular structures. On the other hand, WO98/54233 discloses a polypropylene resin composition in which the content of a high molecular weight polypropylene having an intrinsic viscosity [xcex7] of 9 to 13 dl/g is from 15 to 30% by weight, the molecular weight distribution represented by Mw/Mn is 20 or more, the molecular weight distribution represented by Mz/Mw is 7 or more, and when molded into an injection molded article, the thickness of a skin layer formed on the surface of this injection molded article is 300 xcexcm or more. The described polypropylene resin composition has light weight, has extremely high rigidity and heat resistance and manifests excellent flowability in molding and excellent processability.
However, it is described in, for example, Plastics Age, May, 1980, p. 93, FIG. 2. 27 that when the thickness of a skin layer formed on the surface of an injection molded article increases, ultimate elongation (tensile elongation), namely, toughness lowers, and there is a desire for an improvement of balance between rigidity and toughness regarding the above-mentioned polypropylene resin composition described in WO98/54233.
JP2000-226478A describes a polypropylene resin composition having a molecular weight distribution represented by Mw/Mn of 8 or more and containing a high molecular weight polypropylene having an intrinsic viscosity [xcex7] of 6 to 11 dl/g and a low molecular weight polypropylene having an intrinsic viscosity [xcex7] of 0.6 to 1.6 dl/g. A polypropylene resin composition is described which has light weight, has extremely high rigidity and heat resistance, and shows excellent flowability in molding, generates no cracking in molding and has excellent moldability. However, since the ultimate elongation of the above-mentioned polypropylene resin composition described in an example of JP-A No. 2000-226478 is from 8 to 16%, there is desired an improvement in the balance between rigidity and toughness.
As described above, there is a desire for the development of a polypropylene-based resin composition which, when molded into a molded article, manifests a low tendency of generation of flow mark on the surface of the molded article, namely, has excellent appearance properties such as high die swell and practically no-problematical generation of granular structures, and is excellent in balance between rigidity and toughness.
An object of the present invention is to provide a polypropylene-based resin composition which, when molded into a molded article, manifests a low tendency of generation of flow marks, namely has a high die swell, has excellent appearance such as little generation of granular structures, and the like, and is excellent in balance between rigidity and toughness, to a process for producing the same, and to an injection molded article made of the same.
Namely, the present invention relates to a polypropylene-based resin composition comprising 0.5 to 10% by weight of a propylene-based polymer component (A1) obtained by polymerizing a monomer mainly composed of propylene, having an intrinsic viscosity [xcex7]A1 measured in tetralin at 135xc2x0 C. of 5 dl/g or more and a melting peak temperature TmA1 measured by a differential scanning calorimeter (herein-after, referred to as xe2x80x9cDSCxe2x80x9d.) of 130 to 160xc2x0 C., and 90 to 99.5% by weight of a propylene-based polymer component (A2) obtained by polymerizing a monomer mainly composed of propylene, having an intrinsic viscosity [xcex7]A2 measured in tetralin at 135xc2x0 C. of less than 5 dl/g, a process for producing the same and an injection molded article made of the same.
The present invention will be described in detail below.